High purity air and other gases such as nitrogen and hydrogen are often required in critical applications. Such critical applications include laboratory instrumentation, breathing air systems, hospital compressed air, pharmaceutical processing, and electronic parts manufacturing air used for cleaning computer boards. In laboratories, such as for automotive emission testing, pure air is required as the oxidizer in flame ionization detectors (FID's), as the calibration gas for infrared detectors (IRD's), and as the carrier gas in gas chromatography analyzers. Purified air is also required in cryogenic separation apparatus used to produce liquid oxygen and nitrogen to prevent riming of the heat exchangers with carbon dioxide and water, and to prevent product contamination with oil vapors.
In the past, complex systems were generally needed to produce purified air. These systems often included high temperature catalytic converters that were very expensive. The product air from prior art systems typically met the following contamination limits:
Inlet ConcentrationOutlet ConcentrationWater vapor20,000 ppmv≦1.0 ppmvHydrocarbons   20 ppmv*≦1.0 ppmv*Nitrogen oxides   5 ppmv<0.1 ppmvCarbon dioxide  400 ppmv≦5.0 ppmvCarbon monoxide   20 ppmv≦1.0 ppmv(*as methane equivalent)
Analytical instruments have been advanced and sensitive in recent years and are able to detect lower levels of contaminants. With the improved dectectability, air and other gas purity requirements have become more stringent. For example, breathing air requirements are constantly being increased. Overall, the result is the desire for improved quality, lower contaminant concentration, in air and other gases.
For example, a newly developing field, fuel cells, particularly those using hydrogen as a fuel, require relatively low levels of contaminants in both the hydrogen source and the air stream used for oxidizing the hydrogen. Generally, at least one of the air or the hydrogen fuel, or both, must have very low levels of contaminants, in order to maintain the high efficiency of the fuel cell and to prevent contamination of the fuel cell materials. Contaminants, including sulfur oxides (SOx) and nitrogen oxides (NOx), hydrogen sulfide and volatile organic compounds (VOC's), dramatically reduce the efficiency of fuel cells and cause their deterioration. Current air purification systems are not adequate to meet the needs of the modern industrial requirements. Usually, the hydrogen and air or oxygen is provided from cylinders or other tanks, the gas having been purified to acceptable levels.
Alkaline or PEM fuel cells, which have been used in applications such as the Space Shuttle, are particularly prone to various contaminants, such as hydrocarbons and nitrogen oxides (NOx). Additionally, the efficiency of alkaline fuel cells is greatly reduced, if not completely destroyed, by the presence of carbon dioxide (CO2) in the oxidant stream. Alkaline fuel cells have prospered in applications such as space because purified compressed oxygen has been used.
As the knowledge of fuel cells increases, fuel cells are moving out of the laboratory and the controlled environment. Operation of fuel cells, in uncontrolled environments, is particularly detrimental to the fuel cell. Various filtration arrangements have been proposed for fuel cells systems. See, for example, U.S. Pat. Nos. 6,432,177 and 6,638,339 (Dallas et al.), U.S. Pat. No. 6,780,534 (Stenersen et al.), U.S. Pat. No. 6,783,881 (Stenersen et al.), and U.S. Pat. No. 6,797,027 (Stenersen et al.), which provide various solutions for air purification for fuel cells. Additional and alternate filtration arrangements would be beneficial.